Letters travel along a large conveyor belt at the Main Post Office in Chicago, Ill. last December. / By Brian Kersey, Getty Images

by Donna Leinwand Leger, USA TODAY

by Donna Leinwand Leger, USA TODAY

The U.S. Postal Service expects robust online shopping to boost package shipping 20% over last year, Postmaster General Patrick Donahoe said Thursday.

Thanksgiving and New Year's Eve is the Postal Service's busiest time of year. Mail carriers will deliver nearly 18 billion cards, letters and packages during that time, Donahoe said. The Postal Service projects it will process 658 million pieces of mail on Dec. 17 alone, 130 million more than it processes on an average day.

Beginning Dec. 4, the National Operations Center will have staff working 24 hours a day to navigate mail movement around the world, including 30 million pounds of military mail destined for people serving overseas.

As e-mail has replaced letter-writing, mail volume has decreased sharply, driving the Postal Service into debt. First-Class Mail volume dropped from 103.7 billion letters in 2001 to 78.2 billion letters in 2010. The increase in package volume will not close the gap, Donahoe said. "You really can't make the difference up in packages," he said.

The Postal Service will continue with its five-year plan to consolidate mail-processing facilities and cut hours at some post offices, Donahoe said. The Postal Service has also asked Congress to allow it to deliver the mail five days a week instead of six and to refund overpayments to its pension plans.

The Postal Service has printed. 2.5 billion holiday stamps, and Donahoe urged people to mail greeting cards this year.

"Greeting cards mean so much to people," he said. "Technology is no match for the mail."

Donahoe also commended letter carriers who continued to deliver mail even as Superstorm Sandy struck. Seeing a letter carrier in a neighborhood is a sign "that life is going back to normal," he said.

The Postal Service also announced its holiday "mail by" dates to ensure mail and parcels arrive by Christmas: